A method and apparatus for securing and releasing an underwater flexible line (30) within a channel (20) is disclosed. The channel (20) is formed on the support member (22) of an offshore structure such as the leg of a fixed offshore platform of the tension member of an offshore riser buoy system. The apparatus comprises a sled (22) which advances relative to the channel (20) on a track system. The sled (32) includes an aligning system for orienting and seating the line within the channel (20) as it advances along the length of the structure. Initially, the line is attached to the upper end of the offshore structure, preferably at or above the water surface and allowed to hang freely to the sea bed. Thus, the line is adjacent the channel and readily available for proper aligning and seating within the base of the channel. After the sled aligns and seats the line within the channel, it closes the channel securing the line by means of latches (34) pivotally mounted to one side of the channel. The sled may be advanced along the length of the channel by means of a tow line (50) which is also used to recover the sled once the line has been secured within the channel, or the sled may be advanced by means of gravity.